In one of your articles comparing cameras with different sensor sizes you said something like: comparing "crop sensor" camera to a crop of a FF sensor isn't fair. I couldn't find any place where you explained that, so why is it?

I'm currently evaluating a Canon 7d to my 5dII for the purpose of extending the reach of my 600/1.4x combination to about 1300mm for those especially uncooperative birds that are so common, so that's exactly the comparison I need - is it worth it to use the 7d instead of cropping the 5dII?

The weather here in Oregon has been crappy for bird photos lately and I'll have to hope for better lighting and no fog some time soon to do a decent comparison. So far, it looks like the main problem is the gritty noise of the 7d, even at ISO 800, one stop less than where I get good images from the 5dII (and ISO 1600 is my standard setting for the low light conditions I prefer for my images). There also seems to be a focus problem with this body, but I have yet to do a thorough microfocus adjustment, so that may be part of the problem.

Not sure where I said that or in what context, but yes, it's generally better better to use a 7D than crop a 5D MkII to APS-C (all else being equal)

It really all boils down to how many pixels you have in the cropped image. For example:

A 7D image has 18MP, but a 5D MkII image cropped down to APS-C dimensions has about 8MP, so the 7D image should have about 1.5x the linear resolution.

Of course you have to consider things like noise, frame rate, usable ISO range and AF accuracy in the equation, but purely from the point of view of resolution, the higher pixel density of the crop sensor will give you higher resolution than a full frame sensor cropped down to APS-C

To get the pixel density of the 7D sensor, you'd need a 46MP full frame sensor. In that case, cropping would be just as good as shooting with a 7D

Thanks, Bob, that was my assessment, but there are some whacko websites out there that espouse the opposite!

I've decided the the 7d I have is a dud and will be returning it. I'm seeing all of the worst reported problems: no focus; variable focus; non-functional microfocus adjustment; and gritty noise even at lower ISOs.